Tectonic Summary

The earthquake occurred as a result of thrust-faulting on the boundary
between the subducting Australian plate and the overriding Sunda block of
the Eurasian plate. In the region of the epicenter, the Australian plate
is moving about 6 cm/year in a northerly direction against the Eurasian
plate. The direction of relative plate motion is oblique to the northwest
trend of the plate boundary. Earthquakes such as that of November 2
accommodate the component of plate-motion that is perpendicular to the
trend of the plate boundary. The component of plate-motion that is
parallel to the trend of the plate boundary is mostly accommodated by
strike-slip faulting in the interior of the island of Sumatra. The
largest thrust-fault earthquakes in the Sumatran subduction zone in the
last two centuries were that of 1833, which had a magnitude of 8.8-9.2,
and that of 1861, which had a magnitude of 8.3-8.5.

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